What is lua_call?

This library implements a script transformation function along with some
useful utilities to allow you to call Lua scripts from other Lua scripts
in Redis. There are also some Python-side wrappers to aid in calling Lua from
Python, but that’s just a bonus.

Generally speaking, it adds an internal calling semantic that allows you to
pass KEYS and ARGV between your internal Lua scripts in a sane manner.
This allows you, as a developer, to to develop your Lua scripts using better
practices than copying/pasting similar code between scripts.

Limitations

Due to the way we handle calling conventions, you must be careful in how you
use KEYS and ARGV. Because we are not using a full-on parser for the Lua
language, we use a regular expression to discover uses of KEYS and ARGV to
alter. More specifically, we take examples like the following:

Note that we didn’t mangle the KEYS in redis.call(), but if you were to
have the following:

local string = 'this is a string with KEYS and ARGV, oops!'

… we will mangle the string into:

local string = 'this is a string with _KEYS and _ARGV, oops!'

There are other potential corner cases where our name mangling might be
incorrect, and you are advised to keep your usage of KEYS and ARGV to
reading or writing to KEYS and ARGV or to the simple literal strings of
‘KEYS’ and ‘ARGV’ as in redis.call(‘KEYS’, …).

Defining scripts using lua_call

You have a Redis connection during script definition

If you have your Redis connection available while defining your Lua scripts,
you can use the following calls to automatically define and register the
function wrappers in the Python module, automatically load the script into
Redis, and register the function for internal calling inside Redis:

You don’t have a Redis connection during script definition

If you don’t have a connection during script definition, you can omit the
connection argument during definition. In this case, the scripts will not be
registered, so you must later call load_scripts() to register them. The
following is more or less equivalent to the ‘have a connection’ section
above:

Calling scripts defined with lua_call

Assuming that you have defined your scripts using one of the two methods
outlined above, the example module will have functions defined in the module
namespace called return_args() and call_return(). These wrappers around
the script take exactly 3 arguments: a Redis connection, then a list of KEYS
and a list of ARGV that are passed to the called scripts.

Note that while KEYS and ARGV passed from outside Redis are translated into
strings as part of the calling process, internal calls do not change the types
of arguments passed.

How it works

This library takes scripts that you define, possibly including other Lua
script calls, and changes the source code to allow you to actually perform
those calls. Generally speaking, you can think of this as introducing a new
global value in Redis by the name of CALL, which allows you to both register
functions and call those functions. Now, the truth is that there is no new
global value available in Redis Lua scripting, but your scripts will act as
though that is the case.

As an example of what actually goes on, let’s say that we start out with a Lua
script defined as the below, which is from the included example.py:

return CALL.return_args({}, {1, 2, 3, _ARGV})

After our transformation (and applying some source code formatting and extra
comments so you can understand what is going on easier), we get the following
script:

-- We reference either the externally-called KEYS/ARGV or the internally
-- called KEYS/ARGV in locals called _KEYS and _ARGV
local _KEYS, _ARGV;
if #ARGV == 0 or type(ARGV[#ARGV]) == 'string' then
-- Use the standard KEYS and ARGV as passed from the external caller
_KEYS = KEYS;
_ARGV = ARGV;
else
-- Pull the KEYS and ARGV from the table appended to ARGV
_KEYS = ARGV[#ARGV][1];
_ARGV = ARGV[#ARGV][2];
-- We remove the pushed reference to prevent circular references,
-- which can crash Redis if you aren't careful
table.remove(ARGV);
end;
-- push the arguments onto the ARGV table as call stack arguments
table.insert(ARGV, {{}, {1, 2, 3, _ARGV}});
-- fetch the script hash from the name and call the function
return _G[redis.call('HGET', ':registry', 'example.return_args')]();

Generally, there is some header code prepended to your source, KEYS and ARGV
references are changed to _KEYS and _ARGV, and any time you want to make a
call to another script, we append your arguments to the end of the ARGV table,
and pull the destination script name from a Redis-backed function registry.

Early versions of this library required assigning the result of a call to a
local variable before returning, but this is no longer necessary.

Licensing and source code mangling

Technically speaking, this library will alter the Lua script source code that
you pass in order to insert the code that handles internal calls. I do not
consider this purposeful alteration to result in your code being in any way
derived from or related to this library. Your source code remains your source
code, and this library is a utility to aid in your development and maintenance
processes.